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Re: `list-of' macro snippet [regarding Comprehensions]


From: Rivka Miller
Subject: Re: `list-of' macro snippet [regarding Comprehensions]
Date: Sun, 28 Oct 2012 19:20:32 -0700 (PDT)
User-agent: G2/1.0

On Oct 28, 5:49 pm, Rivka Miller <address@hidden> wrote:
> To help focus on the issue
>
> I dont know if they are necessary for the context of the operation of
> the comprehensions macro that I cant get to work in elisp.
>
> The original is below but the indentation does not match the original,
> nor the paren matching in the last two defuns. I tried helping myself
> by various google and groups searches but if there was any
> satisfactory reply, it missed my results.
>
> (defmacro comp ((e &rest qs) 12)
>   (if (null qs) '(cons ,e ,12)          ; rule A
>     (let ((q1 (car qs))
>           (q (cdr qs)))
>       (if (not(eq (cadr q1) '<-))    ; a generator?
>           '(if ,ql (comp (,e ,@q),12) ,12) ; rule B
>         (let ((v (car ql))              ; rule C
>               (l1 (third q))
>               (h (gentemp "H-"))
>               (us (gentemp "US-"))
>               (usl (gentemp "US1-")))
>           '(labels ((,h (,us)           ; corresponds to a letrec
>                         (if (null ,us) ,12
>                           (let ((,v (car ,us))
>                                 (,usl (cdr ,us)))
>                             (comp (,e ,@q) (,h ,us1))))))
>              (,h ,l1)))))))
>
> (defun open-bracket (stream ch)
>   (do ((l nil)
>        (c (read stream t nil t)(read stream t nil t)))
>       ((eq c '|]|) '(comp ,(reverse l) ()))
>   (push c l))
> )
>
> (defun closing-bracket (stream ch) '|]|)
>
> (eval-when (compile load eval)
>   (set-macro-character #\[ #'open-bracket)
>   (set-macro-character #\] #'closing-bracket))
>
> R
>
> On Oct 28, 5:19 pm, Rivka Miller <address@hidden> wrote:
>
>
>
>
>
>
>
> > Discusuion continued from the link
>
> >http://groups.google.com/group/gnu.emacs.sources/browse_thread/thread...
>
> > For those of you who might be interested in this topic. I am including
> > the html source of the two links as well as verbatim text discussion
> > to continue the topic.
>
> > \begin{verbatim}http://web.archive.org/web/20021108231431/http://www.cs.nwu.edu/acade...
> > Lisp Exercises
>
> > Some of these exercises are in addition to or in place of those in
> > Graham or Wilensky.
>
> > SOME (Chapter 8, Ex. 7)
>
> > Define (has-number-p s-exp) to return true if the s-expression is or
> > contains a number.
>
> > > (has-number-p 1)
> > T
> > > (has-number-p 'a)
> > NIL
> > > (has-number-p '(1 (b (2 c) ((3)))))
>
> > T
>
> > Implement this using a simple conditional, recursion, and SOME.
> > Letting SOME do some of the work is more efficient than a pure CAR-CDR
> > recursive solution.
>
> > Wilensky, Ex. 13.2 revised
>
> > Define the macro our-if to have the form
>
> > (OUR-IF test
> >   :THEN exp1 exp2 ...
> >   :ELSE exp3 exp4 ...)
>
> > This does about the same thing as:
>
> > (COND (test exp exp ...)
> >       (T else-exp else-exp ...))
>
> > Almost everything is optional in our-if except the test. Here are some
> > legal forms and their results:
>
> > > (our-if (> 3 1) :then 'ok)
> > OK
> > > (our-if (< 5 3) :else 'ok)
> > OK
> > > (our-if (> 3 1) :else 'oops)
> > NIL
> > > (our-if (> 3 1) :then)
> > NIL
> > > (our-if (> 3 1) :else 'oops :then 'ok)
>
> > OK
>
> > Closures (Chapter 12)
>
> > Define (make-balance initial-balance) to return a function that takes
> > 0 or 1 arguments. If that function is called with 0 arguments, it
> > returns the current balance. If called with 1 argument, which should
> > be a number, it adds that number to the current balance, and returns
> > the new balance.
>
> > > (setq bal (make-balance 100))
> > <a closure object>
> > > (funcall bal 10)
> > 110
> > > (funcall bal -50)
> > 60
> > > (funcall bal)
>
> > 60
>
> > Modifying list pointers (Chapter 15)
>
> > Define (delete-car list) to modify and return list with the first
> > element of list deleted.
>
> > > (setq l '(a b c))
> > (A B C)
> > > (delete-car l)
> > (B C)
> > > L
>
> > (B C)
>
> > Note: it's impossible to destructively delete the only item in a list
> > and turn it into NIL, but delete-car should at least return NIL in
> > that case.
>
> > MAPCAN (Chapter 15)
>
> > Define (collect-numbers s-exp) to return a list of all the numbers in
> > the s-expression s-exp. s-exp may be an atom, a list, or a list of s-
> > expressions.
>
> > > (collect-numbers 1)
> > (1)
> > > (collect-numbers 'a)
> > NIL
> > > (collect-numbers '(1 (b (2 c) ((3)))))
>
> > (1 2 3)
>
> > Implement this using a simple conditional, recursion, and MAPCAN.
> > Letting MAPCAN do some of the work is more efficient than a pure CAR-
> > CDR recursive solution. Don't worry about duplicate numbers in the
> > result.
>
> > Wilensky, Ex. 15.7 revised
>
> > Adding elements to the end of a list is usually inefficient in Lisp:
>
> >     (append list (list item)) is the worst possible approach, because
> > list gets copied every time a new item is added. If you use this form
> > to build a list N long, you'll have done N squared cons's. Imagine
> > doing that for a simple 100-element list!
> >     (nconc list (list item)) doesn't cons, but still gets very slow as
> > the list gets long, because Lisp has to cdr all the way to the end of
> > the list in order to find the last cons cell to modify.
>
> > A classic solution is to create a data structure called a tconc
> > structure (for "tail concatenate"), which holds two pointers to the
> > same list:
>
> >     a head pointer to the whole list, and
> >     a tail pointer to the last cons cell of that list.
>
> > With this data structure, you can add new elements to the end of the
> > list with just a few quick operations, no matter how long the list is,
> > and you can still get the whole list whenever you need it.
>
> > Therefore, your job is to:
>
> >     Define (make-tconc [ list ]) to return a tconc structure pointing
> > to list. If no list is given, a tconc structure for an empty list
> > should be returned.
> >     Define (tconc tconc-structure [item item ...]) to add the items,
> > if any, to the end of the list pointed to by tconc-structure, update
> > tconc-strcture appropriately, and return the new value of the internal
> > list.
> >     Define (tconc-list tconc-structure list ) to add the items in list
> > to the end of the internal list.
>
> > Note that you can get the internal list at any time with (tconc tconc-
> > structure).
>
> > > (setq tc (make-tconc))
> > <tconc structure>
> > > (tconc tc 1)
> > (1)
> > > (tconc tc 2 3)
> > (1 2 3)
> > > (tconc tc)
>
> > (1 2 3)
>
> > Each successive call to tconc should be efficient, no matter how long
> > the internal list has grown. One test of your tconc structure is that
> > it always obeys the following rule:
>
> >     (eq (last head-pointer) tail-pointer)
>
> > Queues (Chapter 15)
>
> > A queue is a standard data structure in computer science, that's very
> > useful whenever you want to take data in the order in which it occurs.
>
> > For example, keystrokes and mouse clicks in an interface need to be
> > handled in the order in which they occur. If they happen too fast for
> > the program to keep up, the program should save the unprocessed clicks
> > in a queue until it can get to them.
>
> > Once you've done the tconc exercise, defining queue functions is easy:
>
> >     Define (make-queue [list]) to create a queue data structure,
> > initialized to the elements of list.
> >     Define (enqueue item queue) to add the item to the end of the
> > queue.
> >     Define (dequeue queue) to remove the first item from the queue and
> > return it. If the queue is empty, nil is returned.
>
> > list-of (an advanced macro)
>
> > [ Inspired by Guy Lapalme's article in Lisp Pointers, Apr-Jun 91 ]
>
> > list-of is macro that simplifies collecting lists of values of
> > expressions. Though this description is long, and the macro is
> > powerful, it's actually quite simple and can be implemented in less
> > than half a page of code.
>
> > The general syntax is
>
> >     (LIST-OF exp generator-or-filter generator-or-filter ...)
>
> > It's easiest to explain by starting with simple examples.
>
> >     > (list-of (1+ x) (x :in '(1 2 3)))
> >     (2 3 4)
>
> > exp is (1+ x) and (x :in '(1 2 3)) is a generator. A generator is
> > anything that has the form (variable :in list). This generator
> > generates three values for x, namely 1, 2, and 3. list-of returns a
> > list of the value of (1+ x) for those three values of x.
>
> >     > (list-of (1+ x) (x :in '(1 2 3)) (oddp x))
> >     (2 4)
>
> > The exp is simply the variable x, the generator is as before, and
> > (oddp x) is a filter. A filter is any expression that doesn't look
> > like a generator. The filter says "keep only those values of x that
> > are odd." Hence, list-of only collects values for x equal to 1 and 3.
>
> > That's it. Any number of generators and filters can be given. They are
> > applied from left to right. If there are two generators, the second
> > repeats itself for every value created by the first, e.g.,
>
> >     > (setq l '(a b))
> >     (A B)
> >     > (list-of (list x y) (x :in l) (y :in ))
> >     ((A A) (A B) (B A) (B B))
>
> > Likewise, the filters apply in order.
>
> >     > (setq l '(1 2 3 4))
> >     (1 2 3 4)
> >     > (list-of (list x y) (x :in l) (oddp x) (y :in l) (evenp y))
> >     ((1 2) (1 4) (3 2) (3 4))
>
> > This collects (list x y) for every x in l that is odd and every y in l
> > that is even. Notice that
>
> >     > (list-of (list x y) (x :in l) (y :in l) (oddp x) (evenp y))
> >     ((1 2) (1 4) (3 2) (3 4))
>
> > returns the same answer, but does more work. Trace oddp to see the
> > difference.
>
> > One special case that follows naturally:
>
> >     (list-of exp) simply returns a list of exp.
>
> > Our final examples are primarily for hard-core computer science types.
> > You can skip them if you wish. Neither are particularly efficient, but
> > they show the power of list-of.
>
> > To define a function that gets all the permutations of a list:
>
> >     (defun perms (l)
> >       (if (endp l)
> >           (list '())
> >           (list-of (cons a p)
> >              (a :in l)
> >              (p :in (perms (remove a l :count 1))))))
>
> > The list-of collects "(cons a p) for every a in l and every p in the
> > permutations of l with a removed."
>
> > To define a form of quicksort function:
>
> >     (defun qsort (l)
> >       (if (endp l)
> >           '()
> >           (let ((a (car l)) (x (cdr l)))
> >             (append (qsort (list-of (y :in x) (< y a)))
> >                     (list a)
> >                     (qsort (list-of (y :in x) (>= y a)))))))
>
> > This splits l into those elements that are less than the car of l, and
> > those that are greater, sorts each sublist recursively, and joins the
> > results
> > \end{verbatim}
>
> > \begin{verbatim}http://web.archive.org/web/20010911002454/http://www.cs.nwu.edu/acade...
> > Macro Exercises
>
> > defrule - a simple rule definer
>
> > Define the macro defrule. The calling format is
>
> > (defrule name
> >   test test test ...
> >   => action action action)
>
> > where test's and action's are expressions. This defines a rule called
> > name.
>
> > Define the function run-rule to take a rule name and "run" it. Running
> > a rule is the same as evaluating
>
> > (when (and test test test ...)
> >   action action action ...)
>
> > Example:
>
> > (defrule watch-for-vacuum
> >   (= *air-supply* 0)
> >   => (format t "Hold your breath!"))
>
> > > (setq *air-supply* 0)
> > > (run-rule 'hate-vacuum)
>
> > Hold your breath!
> > NIL
>
> > There should be no eval's anywhere in your code. Hint: see the
> > discussion of thunks on page 46 of AI Programming.
> > with-output-to-list
>
> > Common Lisp has a nice special form for building strings, called with-
> > ouput-to-string. The with-output-to-list macro adds similar
> > functionality to list building. It's calling format is
>
> > (with-output-to-list (fn-name [variable])
> >  expression expression ...)
>
> > This evaluates the expressions with fn-name bound temporarily to a
> > function that takes zero or more arguments, efficiently adds them to
> > the end of the list in variable, and returns the current value of the
> > list. The value of the last expression is returned. Hint: read about
> > the built-in special form flet.
>
> > As with with-output-to-string, if variable is omitted, fn-name adds
> > its arguments to a new empty list, and with-output-to-list returns the
> > final value of that list, rather than the value of the last
> > expression. Here are two examples:
>
> > > (with-output-to-list (collect)
>
> >     (dolist (x '(1 2 3 4))
> >       (when (oddp x) (collect x))))
> > (1 3)
>
> > > (setq *l* '(a b))
> > > (with-output-to-list (collect *l*)
>
> >     (dolist (x '(1 2 3 4))
> >       (when (oddp x) (collect x))))
> > nil
>
> > > *l*
>
> > (a b 1 3)
>
> > To be efficient, with-output-to-list should expand into code that uses
> > the tconc approach to list building (Wilensky, Common LispCraft, Chap
> > 15, Exercise 7, and the file list.exs in the c25/exercises area). You
> > should expect to generate fairly different expansions for the
> > "variable given" and "variable omitted" cases.
> > list-of - a simple iterator
>
> > list-of has the following general calling format:
>
> > (list-of expression form form ...)
>
> > where form is either a generator or a filter. A generator is a list of
> > the form (variable :in expression). A filter is any expression that is
> > not a generator. Generators generate values, and filters accept
> > certain values (by returning true). There can be any number of
> > generators and filters in any order.
>
> > list-of makes a list of all the values of expression created by the
> > generators and accepted by the filters.
>
> > list-of is best explained with a series of examples:
>
> > (list-of x)
> >     returns a list of the value of x. There are no generators or
> > filters.
> > (list-of x (x :in l) (oddp x))
> >     returns "a list of those x's, where x is in l and x is odd."
> > (list-of (* x x) (x :in l))
> >     returns "a list of the square's of x, where x is in l."
> > (list-of (* x y) (x :in l) (y :in l))
> >     returns "a list of the products of x and y, where x and y are in
> > l." For example, if l = (2 3), this would multiply 2 x 2, 2 x 3, 3 x
> > 2, and 3 x 3, and return (4 6 6 9).
> > (list-of (* x y) (x :in l) (oddp x) (y :in l))
> >     returns "a list of the products of x and y, where x and y are in l
> > and x is odd." For example, if l = (2 3), this would multiply 3 x 2,
> > and 3 x 3, and return (6 9).
>
> > This may seem complicated, but in fact list-of can be defined in half
> > a dozen lines, or so. The first expression is put inside a call to
> > list, each filter expands into (when filter ...), each generator
> > expands into (mapcan #'(lambda (var) ...) list), and everything nests
> > recursively. The expansions of the above examples are, respectively:
>
> > (list x)
>
> > (mapcan #'(lambda (x)
> >            (when (oddp x)
> >             (list x)))
> >         l)
>
> > (mapcan #'(lambda (x)
> >            (list (* x x)))
> >         l)
>
> > (mapcan #'(lambda (x)
> >            (mapcan #'(lambda (y)
> >                       (list (* x y)))
> >                    l))
> >         l)
>
> > (mapcan #'(lambda (x)
> >            (when (oddp x)
> >             (mapcan #'(lambda (y)
> >                        (list (* x y)))
> >                     l)))
> >         l)
>
> > Hint: the best way to handle recursive macros is to define a recursive
> > "expander" function, i.e.,
>
> > (defmacro list-of (exp &rest forms)
> >   (expand-list-of exp forms))
>
> > expand-list-of is just a normal function that returns the expansion of
> > a list-of call. The nice thing about it is that it can call itself
> > recursively, something that can be messy to do with macros.
> > \end{verbatim}
>
> > I dont know if they are necessary for the context of the operation of
> > the comprehensions macro that I cant get to work in elisp.
>
> > The original is below but the indentation does not match the original,
> > nor the paren matching in the last two defuns. I tried helping myself
> > by various google and groups searches but if there was any
> > satisfactory reply, it missed my results.
>
> > (defmacro comp ((e &rest qs) 12)
> >   (if (null qs) '(cons ,e ,12)          ; rule A
> >     (let ((q1 (car qs))
> >           (q (cdr qs)))
> >       (if (not(eq (cadr q1) '<-))    ; a generator?
> >           '(if ,ql (comp (,e ,@q),12) ,12) ; rule B
> >         (let ((v (car ql))              ; rule C
> >               (l1 (third q))
> >               (h (gentemp "H-"))
> >               (us (gentemp "US-"))
> >               (usl (gentemp "US1-")))
> >           '(labels ((,h (,us)           ; corresponds to a letrec
> >                         (if (null ,us) ,12
> >                           (let ((,v (car ,us))
> >                                 (,usl (cdr ,us)))
> >                             (comp (,e ,@q) (,h ,us1))))))
> >              (,h ,l1)))))))
>
> > (defun open-bracket (stream ch)
> >   (do ((l nil)
> >        (c (read stream t nil t)(read stream t nil t)))
> >       ((eq c '|]|) '(comp ,(reverse l) ()))
> >   (push c l))
> > )
>
> > (defun closing-bracket (stream ch) '|]|)
>
> > (eval-when (compile load eval)
> >   (set-macro-character #\[ #'open-bracket)
> >   (set-macro-character #\] #'closing-bracket))
>
> > R

I spent a few hours poring over and fixed some of the variables and
backquotes and character codes.

The defmacro is now only nested in one function where it is needed.
Hope, someone can help get it to work and produce some kind of demo
examples.



(defun open-bracket (stream ch)


  (defmacro comp ((e &rest qs) l2)
    (if (null qs) `(cons ,e ,l2)        ; rule A
      (let ((q1 (car qs))
            (q (cdr qs)))
        (if (not(eq (cadr q1) '<-))     ; a generator?
            `(if ,q1 (comp (,e ,@q),l2) ,l2) ; rule B
          (let ((v (car q1))            ; rule C
                (l1 (third q1))
                (h (gentemp "H-"))
                (us (gentemp "US-"))
                (us1 (gentemp "US1-")))
            `(labels ((,h (,us)         ; corresponds to a letrec
                          (if (null ,us) ,l2
                            (let ((,v (car ,us))
                                  (,us1 (cdr ,us)))
                              (comp (,e ,@q) (,h ,us1))))))
               (,h ,l1)))))))


  (do ((l nil)
       (c (read stream t nil t)(read stream t nil t)))
      ((eq c '|]|) `(comp ,(reverse l) ()))
  (push c l))
)

(defun closing-bracket (stream ch) '|]|)

(eval-when (compile load eval)
  (set-macro-character #\[ #'open-bracket)
  (set-macro-character #\] #'closing-bracket))






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